1. Field of the Invention
The present invention relates to a molten carbonate fuel cell, particularly its cathode, and to a method for producing the same.
2. Description of the Prior Art
Molten carbonate fuel cells generally comprise a pair of gas-diffusion electrodes with their current collectors, a positive electrode (cathode) and a negative electrode (anode), an electrolyte which is retained in a porous body provided between both the electrodes and makes contact with both the electrodes and a cell housing to physically retain the cell components.
As a material for the cathode of the molten carbonate fuel cell of this type, a metal oxide represented by the formula Li.sub.x Ni.sub.1-x O (0&lt;x&lt;0.05) has long been employed. In most cases, the raw material of nickel powder or nickel oxide powder is first formed into a state of green tape by means of a doctor blading method and the like, then optionally sintered, and thereafter incorporated in a cell. It is thereafter completed into a porous electrode plate comprising a metal oxide represented by the formula: Li.sub.x Ni.sub.1-x O (0&lt;x&lt;0.05) in the cell, by being doped with lithium supplied from the electrolyte, in other words, by an in-situ oxidation and lithiation of the raw material.
At a step of configuring an operable cathode, namely at the time of so-called "starting procedure of the cell", a gas for preventing oxidation, such as carbon dioxide, an inert gas or a reducing gas is usually introduced with an optional humidifying into a site of the anode, and a mixed gas of air with carbon dioxide (for instance, air:carbon dioxide:=70%:30%) is supplied into a site of the cathode of the cell. In this case, a carbon dioxide partial pressure of the introduced gas is 0.1 or higher whereas an oxygen partial pressure is 0.2 or lower. The resultant metal oxide of the composition Li.sub.x Ni.sub.1-x O obtained in this manner by an in-situ lithiation at the time of starting procedure of the cell usually has a value of x which is smaller than 0.05.
The electrode comprising a metal oxide represented by the formula Li.sub.x Ni.sub.1-x O (0&lt;x&lt;0.05) has a high activity for an oxygen reducing reaction which is cathode reaction of the molten carbonate fuel cell and a relatively high conductivity, and thus has widely been employed as a material for the cathode in general.
The nickel oxide and the metal oxide of the composition Li.sub.x Ni.sub.1-x O (0&lt;x&lt;0.05) are however dissolved in the molten alkali metal carbonate which is the electrolyte of the molten carbonate fuel cell as ions including nickel. If it were a simple dissolution, an elution of the nickel from the cathode into the electrolyte would be stopped at a time point when a saturation solubility was reached. The nickel eluted into the electrolyte and existed therein as ions will however be reduced and deposited in the electrolyte as metal nickel, if it is exposed to a reducing atmosphere in the vicinity of the anode. Once it is deposited, the nickel contained in the cathode will continue to be eluted.
If the elution of nickel from the cathode is continued, the cathode will gradually lose its weight, thereby to invite an increase in contact resistance. Further, it will become difficult to maintain a fine structure which is optimum for maintaining a three-phase zone of gas-electrolyte-electrode. Moreover, the nickel deposited in the electrolyte will finally bring a short-circuiting between the cathode and anode. In this manner, the elution of nickel from the cathode is considered to be a great detrimental factor dominating the loss of service life of the cell.
In order to cope with this difficulty, an attempt for suppressing the solubility of nickel has been investigated by adding an alkaline earth metal carbonate or the like to the electrolyte. Although the solubility of nickel can be suppressed to about 1/2-1/3 of its initial value by this means, the service life of the cell is scarcely prolonged and a sweeping solution for obviating the detrimental factor cannot be reached.
In recent years, there is a continued search in this art for substitution materials such as LiFeO.sub.2. Although these materials have a scarce elution into the electrolyte and re-deposition at the vicinity of the anode, they however have a low activity for the oxygen reducing reaction and a low conductivity, and hence, a sufficient cell performance cannot be obtained with these materials.